1. Field of the Invention
The present invention relates to the transmission of electrical carrier signals, and to the recording and reproduction of sound, and in particular to the reproduction, the re-recording and the playback of previously recorded sound signals.
2. Description of the Prior Art
In playing back a sound recording, one hears not only a reproduction of the program component, but also the undesirable extraneous noise components that are caused by imperfections in the original recording and in the storage medium, damage to the storage medium, and inherent defects inadvertently introduced by the metal parts used in the manufacture of the storage medium. Such noises commonly are designated by such terms as "surface noise", "crackle", "ticks", etc. These noises are generally of a first type of an audibly continuous nature distributed fairly uniformly throughout the audio frequency range and known as "white" noise and of a second type which are transient sounds of short duration and high amplitude relative to the signal. (As used herein, the term "program component" refers to the electronic signal corresponding to the music, message or other sound intended to be transmitted to the listener or receiver; the term "noise transient" refers to the undesirable impulses and similar disturbances which may be imposed on the program material; and the terms "total input signal" or "carrier signal" refer to the total signal being transmitted-- i.e., program component plus noise.)
Radio broadcast signals also may contain some additional noise components caused by disturbances in the transmission or reception, commonly called "static". Static, also, typically consists of pulses of short duration and high amplitude.
Previous processes in reducing unwanted noises in sound reproduction have relied on restricting the frequency range as far as is feasible for the purpose or application. Typically, the higher frequency components of the noise, usually referred to by such terms as "surface noise", "crackle", "ticks", etc. are suppressed by limiting the reproduction of the higher frequencies throughout the playback, usually at some sacrifice to the fidelity of reproduction of the signal. Some sophistication in this process was introduced by Harmon H. Scott whose Dynamic Noise Suppressor used electronic means automatically to alter the width of the frequency passband in record reproduction in accordance with the loudness of the music. The Scott Suppressor takes advantage of the phenomena that the relative sensitivity of the ear to various frequencies varies with the loudness of the sound, and that louder passages of music and other audio signals have a correspondingly improved masking effect on the surface noise perceived from the recording; so that, in louder passages a wider frequency range is feasible in reproduction than is the case in the reproduction of the quieter passages for similar listening comfort and perceived tone quality. The operation of the Dynamic Noise Suppressor is, basically, a rather gradual one, especially in closing down the frequency range after a loud sound, lest a too evident change in background noise becomes audible. The Dynamic Noise Suppressor, thus, is incapable of suppressing transient noises save in a generalized manner by its relatively long term modification of the pass band; it copes much more successfully with continuous "white" noise than with noises of a pronouncedly transient nature. These same observations apply to such recent developments and extensions of the Scott principle as are embodied in U.S. Pat. Nos. 3,678,416 (Burwen) and 3,803,357 (Sachs). A different method of subjectively reducing noise is described in U.S. Pat. No. 3,275,326 (Welch), in which two identical recordings playing simultaneously are auditioned.
Compressor-expander systems are well known in the art of sound recording and reproduction. These systems function by compressing the dynamic range in the recording process, and then by expanding the dynamic range in the playback process in an amount and manner complementary to the compression. The following U.S. patents disclose various compressor-expander systems: U.S. Pat. Nos. 3,655,355 (Dolby); 3,729,693 (Dolby); 3,732,371 (Burwen); 3,813,559 (De Boer); 3,815,039 (Fujisawa); 3,828,280 (Dolby); 3,829,715 (Van Sluys); and 3,846,719 (Dolby). These systems are only effective in reducing noise in those sound recordings which are specifically pre-encoded for such systems.
There are also noise reduction systems wherein noise transients are removed from transmitted signals by blanking operations wherein the entire signal is blocked from transmission for an interval intended to be co-extensive with a noise transient, and the charge on a capacitor replaces the signal during the blanking interval. Thus, U.S. Pat. No. 3,668,416 (Burwen) described a circuit which utilizes a capacitor charged by low frequency energy from the input signal, and which substitutes the capacitor charge for the signal when a tick is detected. In U.S. Pat. No. 3,978,412 (Frerking), a switching device is described which "clamps" the audio signal for a predetermined blanking period when a noise transient is detected. A blank is instituted and a capacitor engages to load down and replace the signal emanating from the noise detector. Patent application Ser. No. 848,576, Nov. 4, 1977, by the present Applicants discloses a clipping process and system for reduction of noise transients.
Diversity receivers used in the reception broadcasts represent a specialized field of improvement of the program component of a carrier signal in which a choice is made of the best of two or more separate receptions of the same broadcast signal. The following U.S. patents disclose various diversity receiver systems: U.S. Pat. Nos. 2,969,718 (McKesson); 3,997,844 (Jayant); 2,604,587 (Lyons); 3,401,340 (Cronberg, Jr.); and 3,475,686 (Holt, Jr.).
A prior patent of the present applicants, U.S. Pat. No. 4,155,041, concerns the reduction of noise in a carrier signal that is caused by discrete noise transients. Novel methods for suppressing these are incorporated in the system. One method takes advantage of the circumstance that imperfections in a recording, such as imperfections in the record material, defects in manufacture, damge, dirt, wear, mildew, etc., which may introduce transient noises into the reproduction of the recording, are not identical on opposite sides of the record groove, whereas, in the case of a monophonic, lateral-cut phonograph record, the program component is identical on the opposite side walls of the groove. Our prior invention reduces transient noises in the reproduction of the recording by a continuous process of switching the reproduction to the one of the two groove walls having the lesser noise content. An additional embodiment of our prior invention provides means by which the switching process can be a three-way one between either of the groove side walls or the sum signal obtained by mixing the signals of the two side walls. It is this last described switching process that is the more effective one in reducing transient noises in the reproduction of lateral-cut monophonic records as, normally, in their reproduction with stereophonic playback equipment, the two channels of the reproduction are equally mixed, and this has the desirable result that effects caused by the vertical component of the stylus motion, such as pinch effect distortion and the vertical components of turntable rumble and surface noise and, also, noises due to vertical imperfections in some records, are cancelled out. In light of this cancellation of purely vertical noise, there are occasions when the combined or summed signal of the two channels has a lower net noise level than either of the component signals in the respective channels.
Other noise reduction and signal improvement systems are disclosed in U.S. Pat. Nos. 2,736,711 (Hanson); 2,912,571 (Jacobson); 3,171,901 (Clemency); 3,394,235 (Schott); 2,977,579 (Mullen); 3,800,164 (Miller); 2,261,951 (Bloch); 3,180,396 (Schroeder); 3,989,897 (Carver).